In recent years, memories have been developed, for example for computers, which have been increasingly compact, but there is, nevertheless, a demand for still more compact memories. An object of the present invention is to provide such a memory, and in particular a memory employing a Langmuir-Blodgett film (hereinafter referred to as an L-B film).
Before the invention is described in detail an outline will be given of the nature and properties of L-B films.
Many molecules having a hydrophilic and hydrophobic end, for example long chain fatty acids, form insoluble monolayers at an air-water interface. The packing in the monolayer may be controlled by the application of surface pressure through barriers and the equation of state of the film is given by the surface pressure-area isotherm. When an appropriate substrate, for example glass, silicon or indium phosphide, is dipped through the air-water interface then one monolayer may be transferred to the substrate each time the interface is traversed. A film of great perfection can thus be built up a single monolayer at a time. It has been demonstrated that it is possible to build up extremely precise supermolecular structures consisting of fatty acids, long-chain dyes and similar molecules for the study of electron and exciton transport. More recently it has been demonstrated that fatty acids with certain substitutes, for example a diacetylene group, may be polymerized either at the air-water interface or after the film has been prepared.
Charge and energy transport in L-B films will now be summarized.
(a) Electron tunneling.
Monolayers of fatty acids with varying chain lengths, and hence varying thickness, have been prepared as a sandwich between conducting aluminium layers. The electrical conductivity of such films has been demonstrated to decrease logarithmically with increasing monolayer thickness. This is the result which would be expected if the currents were due to electrons tunneling through the dielectric monolayers. With some reservations this view is generally accepted as is the conclusion that these experiments demonstrate the remarkably perfect quality of the monolayers.
(b) Exciton transfer.
Monolayers of dye substituted fatty acids commonly exhibit the characteristic absorption and fluorescence spectra of the isolated dye. Detailed investigations have been made of energy transfer from one type of dye in one monolayer to a second type of dye in neighbouring or more distant monolayers. If for example a sensitizer dye S which absorbs in the UV and emits in the blue is incorporated in an L-B film assembly with an acceptor dye A which absorbs in the blue and emits in the yellow then considerable energy transfer can occur. Under UV illumination the blue fluoresence is partially quenched by the presence of dye A and yellow fluorescence appears. The relative quenching of dye A fluorescence depends upon the proximity of dye S in a manner predicted by the classical electric dipole model.
(c) Photoinduced electron transfer.
Electron as well as exciton transfer has been observed between different chromophores in multilayer assemblies. In this case when the photon is absorbed an electron is transferred from one molecule acting as the donor D to the second acting as acceptor A. Quenching of fluorescence is observed in monolayer assemplies if donor and acceptor are in the same monolayer or at the hydrophilic interface between adjacent monolayers. When D and A layers are separated by a single fatty acid monolayer it has been possible to observe this transfer as a photocurrent.
(d) Compensated photoinduced electron transfer.
In general the photoinduced electron transfer D - A is reversible and in the dark the electron will return A - D. The reverse process can be inhibited if an electron source molecule ES can supply an electron to the photo-oxidized donor. This possibility has been demonstrated by a monolayer sandwich: ES (leucostearylenblue), D (.omega.-pyrenestearate), fatty acid, A (dioctadecyl-bipyridinium). Under illumination the system acted as an inefficient electron pump.
(e) Photoinduced electron release.
Photocurrents can be generated from a layer of an absorbing molecule located in a film of fatty acid layers if the excited state energy level of the absorber lies close to the potential barrier of the fatty acid layers. It is known that this is possible for the linear conjugated molecule quinquethienyl for which the excited state lies 0.4eV below the potential barrier of arachidate layers.
The potential of the L-B multilayer technique for the fabrication of supramolecular structures has been amply demonstrated, as described above. However, the practical application of structures with photo-excited energy and charge transfer has been inhibited by the poor stability of L-B multilayers primarily composed of fatty acids. This arises from the low melting points of the long-chain fatty acids and the large amplitude molecular motions, which give rise to solid-state phase transitions below the melting points of paraffinic crystals. Thus, the ceiling temperature of fatty-acid L-B multilayers is close to room temperature and they exhibit pronounced ageing, with consequent changes in physical properties, due to molecular re-arrangement within the L-B layers.
One solution to this problem, which has been extensively studied over a number of years, is the inclusion in the monolayer-forming molecules of reactive units capable of producing polymer chains within the layer. Such reactions can occur in L-B layers since the molecular packing within each layer brings the reactive units into close contact. Molecules containing double bonds were studied first, e.g vinyl stearate and octadecyl methacrylate. polymerization was observed with UV and electron beam irradiation but the materials were found to oxidise readily, so that all film preparation has to be carried out in an inert atmosphere, and the dimensional changes on polymerization gave a rather imperfect product.
The solid-state topochemical polymerization of certain di-substituted diacetylenes has been known for some time. This polymerization is insensitive to normal atmosphere and there followed development to investigate the properties of L-B films made from fatty-acids containing diacetylinic units. The topochemical polymerization was found to occur under UV in irradiation in air and the dimensional changes were sufficiently small that the final films were as perfect as the initial monomer films.
The stability and quality of polydiacetylene L-B multilayers has been shown by their inclusion in MIS devices. Although the conditions for the formation of pin-hole free L-B monolayers are more stringent than those for the unsubstituted fatty acids they are now well documented in the literature so that routine fabrication is possible. The ceiling temperature of polydiacetylene L-B films has not been critically determined. Values in excess of 200.degree. C. are to be expected since this is the regime in which polydiacetylene crystals are observed to decompose. In one case the first stage of this decomposition has been identified as cleavage of the bulky, reactive sidegroups, probably initiated by absorbed oxygen. This suggests that diacetylenes with less reactive paraffinic sidegroups are likely to decompose at higher temperatures. Ageing of the polymerized films should be negligible since on polymerization the paraffinic side chains are locked in place by the polymer chains, which prevents any translational motion either in or out of the plane of the film. This is revealed most dramatically by the disappearance of phase transitions in both the pure acids and their salts. It should be emphasised that the incorporation of polydiacetylene chains into L-B multilayers offers benefits in addition to providing more durable films. These derive from the properties of the PDA chain, which is a wide band-gap wide band semiconductor with strong electron-hole interaction. Thus the PDA chains can play an active role in the photo-excitation of energy and charge transfer through L-B multilayer structures.
The electronic and vibrational excitations of the conjugated polydiacetylene backbone are now very well understood. Optical absorption and reflection spectroscopy have demonstrated the existence of an exciton state on the backbone at approximately 2eV while photoconduction measurements have shown that the conduction band lies 2.4eV above the valence band. Resonance Raman spectroscopy has revealed that only a few phonons on the backbone are strongly coupled to the exciton. The most prominent of these are at 2100cm.sup.-1 and 1500cm.sup.-1 ; lattice dynamical analysis of the backbone has shown that the former mode primarily involves distortion of the triple bond while the latter involves the double bond.